Method using membrane bioreactor with reduced air scour requirements

ABSTRACT

In a sewage treatment plant with a membrane bioreactor (MBR), raw untreated or partially treated wastewater (influent) and/or mixed liquor in the intensified process is superoxygenated. In a preferred form of the process the influent is superoxygenated such that membrane air scouring requirements are reduced. Influent may be fed to a swing zone where denitrification and nitrification occur simultaneously through process control. In one embodiment superoxygenation is conducted in an internal recycle stream of the mixed liquor, with oxygen content supersaturated preferably to over 300 parts per million. Due to the active evolution of oxygen gas from the supersaturated stream, occurring preferably by seeding the supersaturated stream with air bubbles, the supersaturated oxygen can serve the dual purposes of meeting process oxygen demands and providing part of air scouring requirements for the membranes.

BACKGROUND OF THE INVENTION

This invention concerns sewage treatment processes and systems, andparticularly involves a method and system used in a plant havingmembrane bioreactors that intensify a treatment process, for loweringair scour requirements through superoxygenation of raw wastewater feed(influent) or mixed liquor in the MBR tank, while providing neededoxygen for the intensified process. The method of oxygenating theinfluent can also break down refractory organics, improving processefficiency.

Membrane air scouring is necessary to remove the solids that accumulateduring filtration and generally accounts for 25%-75% of total overallsystem energy demand. Due to inefficient diffusers and oxygen transferrate (OTR) limitations, much of the oxygen contained in scouring air iswasted to the atmosphere instead of being used for biological processes.Moreover, increasing mixed liquor suspended solids (MLSS) concentrationfor the purposes of reducing process volumes further reduces OTR,requiring systems to be larger and more complicated to operate. Thisinvention uses a stream of supersaturated influent and/or mixed liquorto reduce the amount of air required for membrane scouring purposes andprovide all or most of the biological process oxygen. This inventionreduces or eliminates the need for diffused aeration.

Most submerged MBR technologies use diffused aeration to scour awaymaterials that accumulate on membrane surfaces during filtration (socalled jet aeration is an alternative). Generally, coarse bubbleaeration has been shown to be the most efficient means of air scouringbut it is the least efficient means of delivering oxygen to thebiological process. Given increasing energy costs, some manufacturershave turned to fine-bubble diffusers for better oxygen delivery and toprovide air scouring, with attendant maintenance issues and increasedequipment costs. Regardless of the aeration technology used to providescouring air, the oxygen transfer rate (OTR) limits how much abiological process can be intensified (volume reduced) as diffuserperformance drops off precipitously with increasing mixed liquorsuspended solids concentrations (MLSS).

Prior oxygenation systems have been proposed that introduce (pure)oxygen into mixed liquor through agitation or mixing. However, thesetechnologies introduce oxygen into other zones in the biological flowsheet and not directly into the influent or the MBR tank, and none hassuggested utilizing the supersaturated oxygen to meet some of themembrane air scour requirements.

Due to increased energy costs, it would be greatly advantageous ifsupersaturation of oxygen could be used in an MBR system for bothprocess enhancement through increased oxygen content and for scouringaeration, to thereby reduce separate air scour requirements.

SUMMARY OF THE INVENTION

In accordance with this invention, by supersaturating an internallyrecycled stream of mixed liquor or influent (pre-treated sewage) withoxygen and introducing a small amount of pre-formed seed bubbles (ordiffuser air bubbles), the need for diffused air can be reduced and theprocess made more efficient. Using the oxygen contained in a stream ofsupersaturated mixed liquor or influent in place of diffused air forscouring and process needs can reduce total system energy requirementsby 25%-50%, increase space efficiency by 25%-50%, simplify process flowsheets, reduce system maintenance requirements and potentially partiallyconvert refractory organics into readily biodegradable materials.

MBR technologies allow for what is called process intensification.Activated sludge processes are intensified as the concentration ofactive biomass is increased and proportionally process volume isdecreased. Using membranes instead of sedimentation to remove solidsallows for concentrations 3-5 times higher than for conventionaltechnologies. However, further intensification is primarily limited byoxygen uptake rate (OTR) required for the biological process. OTR is afunction of oxygen saturation, which at ambient conditions is about 10ppm, and diffuser performance. Supersaturating process influent, aninternal recycle stream or both overcomes both of these limitations andcan increase the intensification factor by 25%-50% depending on processconditions (e.g. food to mass ratio).

Typically conventional activated sludge plants run at MLSSconcentrations around 3,000 mg/l. Typical sMBR plants run at 10,000 mg/lwhen optimized for energy and given diffuser performance limitations athigher solids concentrations. The preferred range of MLSS concentrationsis 20,000 mg/l to 30,000 mg/l for this invention.

Influent mixed liquor pursuant to the invention can be supersaturated(relative to ambient oxygen saturation) with oxygen to more than 50 ppm,or a range of about 250 to about 300 ppm. Superoxygenation can be toover 300 ppm (generally requiring about 100 p.s.i. pressure). However,added oxygen stays in solution or evolves as fine micro-bubbles that arenot effective for air scouring purposes due to low rise velocity. It isgenerally accepted that a rise velocity on the order of 1.5 ft/sec willinduce the most efficient hydraulic flow regime for membrane scouringpurposes.

The invention supersaturates (relative to ambient), under pressure, apumped influent and/or an internal recycle (IR) stream (i.e. from themembrane tank back to the same tank) with oxygen and subsequentlyco-mingles that stream with a small amount of pre-formed (seed) bubbles.These can constitute, for example, less than 10% of the volume of theoxygen bubbles emerging from solution. The co-mingled stream is thendischarged into a reactor beneath submerged membranes where dissolvedoxygen evolves as larger bubbles that rise at a sufficient velocity toinduce air scouring, in a first embodiment as described below.

A second embodiment uses supplemental diffused aeration combined withco-mingled bubbles and a third embodiment uses only diffused bubbles forseeding purposes. Seed bubbles, however contacted with the oxygensaturated IR, allow evolving oxygen to form larger bubbles withsufficient rise velocities to induce effective membrane air scouring.

It is thus among the objects of the invention to improve efficiency ofMBR systems in treatment plants by combining superoxygenation ofinfluent or MLSS with the air scouring function. These and otherobjects, advantages and features of the invention will be apparent fromthe following description of preferred embodiments, considered alongwith the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a prior art membrane filtration system in atreatment plant, with air scour.

FIG. 2 is another diagram showing a typical MBR process flow sheet.

FIG. 3 is a further diagram showing an embodiment of a system of theinvention.

FIG. 4 is a side view showing a static air eductor for mixing air intoan oxygen-saturated mixed liquor stream.

FIG. 5 is a diagram showing a second embodiment of the system of theinvention.

FIG. 6 is another schematic diagram showing a variation of the system.

FIG. 7 is a diagram showing a further embodiment of the invention.

FIG. 8 is another diagram showing another embodiment of the invention.

FIG. 9 is a schematic diagram showing another embodiment of theinvention, with supersaturation of oxygen in an influent stream.

FIGS. 10 and 11 are schematics showing variations of the embodimentsshown in FIG. 9.

FIG. 12 is a schematic diagram showing a variation of a system withinfluent being supersaturated.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows, in a simple diagrammatic form, a typical prior artmembrane filtration system or MBR (membrane bioreactor) system, as usedin a sewage treatment facility. The figure indicates a system 10 with amembrane filtration zone 12 that contains a series of membranefiltration units. The submerged membranes receive mixed liquor or MLSS(mixed liquor suspended solids) entering the zone as indicated by thearrow 14, and liquid filtrate or permeate is indicated exiting the tankor zone with the arrow 16. As also is typical, air scour is shown at 18,with a blower 20 releasing a multiplicity of vigorously rising airbubbles from below the membranes 13. The air scour removes sludge fromthe surfaces of the membranes as the filtration progresses, and alsoprovides oxygen for the microbial action that occurs in the zone 12.

FIG. 2 shows a prior art process involving membrane filtration, atypical MBR process flow sheet, with an anoxic zone or tank 22, anaerobic zone or tank 24 and a membrane filtration zone or tank 26(which, in many prior art systems, can be a sedimentation tank ratherthan an MBR tank). One recycle of MLSS is shown at 28, from the MBR zone26 back to the aerobic zone 24. Another recycle stream is shown at 30,from the aerobic zone 24 back to the anoxic zone 22 (the letter Pindicates a pump). Process air is shown being admitted to the aerobiczone at 32, and air scour is shown applied beneath the MBRs at 34 (theletter B indicates a blower). Influent wastewater is shown entering thesystem at 36, and effluent permeate from the membrane filtration isshown exiting the system at 38. Whether MBR or sedimentation is used inthis zone 26, solids must be removed periodically or continuously forfurther treatment usually in digesters in the solids side of a treatmentplant.

This is a typical denitrification process at the liquid side of a sewagetreatment plant. In the anoxic zone 22 the microbes acting on the MLSSare given access to a limited supply of oxygen, so that the microbesutilize the oxygen from nitrates in the wastewater being treated,thereby giving off nitrogen gas. In the aerobic zone 24, process air 32provides plenty of oxygen and the ammonia in solution is broken down,with the nitrogen being attached to oxygen to form nitrate. The MLSSrecycle 30 brings much of this nitrate back to the anoxic zone, to bereduced to free nitrogen. In the MBR zone 26, the MLSS is greatlyconcentrated by withdrawal of the permeate at 38, and much of thisconcentrated MLSS is returned via the recycle 28 to the aerobic zone 24.The resulting sludge that is removed from the zone 26 is relatively lowin nitrogen. All of this is very well known in the art.

FIG. 3 shows a first embodiment of the system of the invention. Here, aMBR reactor zone or tank 40 receives influent MLSS at 42 and dischargesliquid permeate at 44. An internal recycle is shown at 46, in which MLSSis moved by a pump P to be supersaturated with oxygen (“SO”), at 48,under pressure, i.e. beyond ambient pressure saturation level. Thesupersaturated MLSS recycle stream is then reintroduced to the tank 40,but is first “seeded” with pre-formed bubbles PB, at 50. Theoxygen-supersaturated recycle stream with the seed bubbles isreintroduced into the tank below the membrane units 13 and, as explainedabove, the seed bubbles provide a medium for the dissolved oxygen toevolve into bubbles that are large enough to have a sufficient risevelocity to provide air scour for the membranes. Thus, the evolvingoxygen can provide some of the air scour requirement, while the oxygenalso serves the biological needs of the intensified process.

Although FIG. 3 can represent an MBR zone or tank in a system havingother zones for denitrifying the mixed liquor, in a preferred embodimentthe illustrated MBR zone 40 is a simultaneousnitrification/denitrification zone, wherein MBR filtration is constantand the MLSS undergoes alternating periods of aerobic and anoxicconditions. This is achieved by lowering and limiting the oxygen contentof the recycled MLSS (to about 1.0 ppm oxygen) to produce anoxicconditions, even though the superoxygenation and seed bubbleintroduction continue. The rate of recycle pumping can be adjusted ifneeded, to balance the system so that oxygen demand is only met to thetarget limited extent while leaving some oxygen as air scour bubblesthat ultimately go to atmosphere. This simultaneousnitrification/denitrification process is discussed in U.S. Pat. No.6,712,970, and as carried out in an MBR tank, is discussed in U.S. Pat.No. 6,743,362, both patents owned by the assignee of this invention.Thus, in the tank 40 anoxic conditions are created to the point thatmicrobes consume the oxygen in nitrates and thus release nitrogen gas;and aerobic conditions also exist, with adequate oxygen so that furtherbreakdown of ammonia commences. Note that in the embodiment shown inFIG. 3 no additional air is used for air scouring of the MBRs 13, onlythe dissolved and evolving oxygen along with the seed bubbles.

FIG. 4 shows one method for introducing “seed” air bubbles into theoxygen-supersaturated MLSS recycle flow. Oxygen-supersaturated mixedliquor is shown at 52 entering through connections into an eductor 54arranged in line in the recycle flow. The MLSS stream flows through theeductor 54 which has a side air port 56 for venturi-activated drawing inof air, through the end of a tube 58. The eductor is internallyconfigured to draw the air in the form of small bubbles, which mixtogether with the oxygen-supersaturated MLSS flow, emerging at 60. Thisassembly serves as the seed bubble introduction device 50 shown in FIG.3 and also in other drawings.

FIG. 5 shows another embodiment of the invention, similar in manyrespects to the first embodiment shown in FIG. 3 but with some air scourof the membranes 13 introduced, using a diffuser driven by a blower 62.In this case the air scour provided by the evolving oxygen bubbles withseed air is supplemented by diffuser air scour at 64, still at a greatlyreduced air scour flow rate from that typically used in prior artsystems such as in FIG. 1. For example, diffuser air scour in thisembodiment can be reduced by 10%-50% as compared to the typical systemshown in FIG. 1.

FIG. 6 shows a variation of the invention in which the introduction ofseed bubbles is not included. Oxygen supersaturation is shown at 48 inthe recycle stream 46, and this recycle stream is reintroduced into thetank 40 below the membrane units 13, as in previously describedembodiments. Diffuser air scour is again shown at 62, 64. In this case,the air scour, at a greatly reduced rate as compared to prior art,provides “seed” bubbles for the oxygen evolving from solution, and thediffuser air and the evolving oxygen bubbles together provide sufficientair bubbles with sufficient rise velocity to perform air scourefficiently. In this embodiment the air scour rate at 64 is typicallysomewhat higher than in the embodiment shown in FIG. 5.

The system embodiments of FIGS. 5 and 6 can have the influent treateddirectly at the inflow to the MBR zone at 42. This can be alternative toor in addition to the IR oxygenation shown.

FIG. 7 shows a modified system 68 in which a recycle stream 46 a ispumped into back into the MBR zone 40 a as in FIG. 5, but the systemincluding another process zone 70 which receives recycle 71 from the MBRzone 40 a, while also receiving influent 42. In the recycle stream 46 athe supersaturation of oxygen is conducted as shown at 48 and pre-formedseed bubbles are injected as indicated at 50. This recycle is deliveredinto the tank 40 a beneath the membranes 13 as in FIGS. 5 and 6. Theaerobic or swing zone 70 serves as an anoxic and aerobic zone, receivinginput wastewater 42 and the recycle stream 71. Process air for this zoneis shown at 72, introduced by a blower. This process air 72 can bevaried from a high flow rate to a low flow rate, or oxygen content canotherwise be controlled, to maintain in the zone 72 aerobic and anoxicconditions, functioning to remove nitrogen as explained above for otherembodiments. The nitrogen-reduced MLSS then progresses, as indicated at74, into the membrane tank 40 a (which could be a sedimentation tank).Air scour for the membranes is shown at 75, delivered from air diffusersto clean the membranes 13, but at greatly reduced flow because evolvingbubbles from the recycle stream 46 a, i.e. oxygen bubbles which aremixed with the preformed bubbles (PB) perform significant air scour. Thesystem of FIG. 7 has advantages in that in the MBR zone 40 a the MLSShas been thickened, sometimes to about 3%, and process oxygen demandsare higher as reviewed above. Thus, the superoxygenated recycle isimportant for supplying process oxygen, as well as for producing bubblesor air scour as in the other embodiments. An important attendantadvantage is that the supersaturation with oxygen at 48 can becontrolled. The oxygen is introduced into solution under pressure, andthat pressure can be controlled as desired, in order to finely tune theoxygen content in the MLSS in the zone 40 a. This ability for accurateprocess turndown, or “rangeability” of the system, provides an importantprocess control that cannot be achieved with blowers and diffusersnormally used. Diffusers are affected by thickness of the MLSS, and theefficiency of the air injection diminishes with thickening sludge. Thisadvantage also occurs with the above described embodiments as well. Inthis system simultaneous nitrification/denitrification is conducted inthe separate swing zone 70. Liquid permeate is withdrawn at 76. Again,the FIG. 7 embodiment can include oxygen supersaturation at the swingzone influent at 42, along with or as an alternative to the recycle SOinjected at 48.

In the system 80 of FIG. 8 a further modification is made from theearlier embodiments. Separate anoxic and aerobic zones 22 and 24 areincluded. A first recycle stream 82 recycles mixed liquor from and backto the membrane tank 40 b, as in FIG. 7. A second recycle stream 84recycles MLSS from the zone 40 b to the aerobic zone 24, while a thirdrecycle stream 86 recycles MLSS from the aerobic to the anoxic zone 22.Process air 72 provides aerobic conditions in the aerobic zone 24. Acomparison of this figure to the prior art of FIG. 2 will show that thesystems are similar, except that in the MBR filtration (orsedimentation) zone 40 b the MLSS stream in the recycle 82 includessupersaturating the stream with oxygen at 48 and introduction of seedbubbles at 50, which, as explained above, provide accurate systemrangeability for oxygen content in the zone 40 b while also reducing airscour requirements for the membranes. Again, supersaturated oxygen couldbe introduced at 42.

FIG. 9 shows another variation of the system, basically a variation ofthe system of FIG. 3. The difference here is that the influentwastewater stream 42 a, as well as the internal recycle, issupersaturated with oxygen, as indicated at 90. This can be advantageousin controlling the oxygen content in the MBR zone 40. As noted above,pressure can be varied, through an infinite range of variation, tointroduce as much oxygen at 90, as well as at 48, as the processrequires. The influent wastewater at 42 a is much diluted as compared tothe thickened MLSS resulting from the MBR zone 40, so that oxygencontent can be varied using adjustable supersaturation of oxygen at twodifferent stages of the process. The oxygen-supersaturated influent 42 acan be admitted directly beneath the membranes 13, along with theoxygenated and bubble-seeded recycle stream 46, to produce air scour.Although the zone 40 may be a swing zone (simultaneousnitrification/denitrification) as in FIG. 3, it can optionally be simplyan aerobic zone (thus the word “anoxic” is in parentheses).

FIG. 10 is a variation of FIG. 9, with a second recycle stream 92reintroducing MLSS with preformed bubbles as noted at 94. This streamthus supplements the preformed bubbles introduced at 50 to involveoxygen bubbles at a position to scour the membranes. As in FIG. 9, thiszone can be simply aerobic if desired.

In FIG. 11, the system of FIG. 10 is modified to deliver seed bubblesonly via the recycle stream 92, without introducing preformed bubbles inthe recycle stream 46. Again, the preformed bubbles in the recyclestream 92 in both FIGS. 10 and 11 can serve to evolve bubbles from theinfluent stream 42 a as well as from the recycle stream 46. As in FIG.9, this can be simply an aerobic zone if desired.

In a variation, any of the systems of FIGS. 9-11 can receivesupersaturated oxygen only at the influent 42 a if desired, eliminatingthe SO by recycle stream. See also FIG. 12 and the discussion below.

FIG. 12 shows a variation of the system shown in FIGS. 9-11. Here,influent wastewater enters at 42 a, and this influent is treated withoxygen supersaturation “SO”, indicated at 48, and then with preformedseed bubbles “BB”, indicated at 50. In the example system illustrated,an aerobic stage 96 is positioned as a first stage in the liquid sidetreatment, and this stage is followed by an MBR zone 40. As an example,oxygen content can be about 300 ppm as the influent enters the aerobiczone at 42 a. An air blower 97 can be included. The process will take upmuch of the oxygen, and about 100 ppm may remain in the MLSS enteringthe MBR zone at 98. This oxygen will partly be taken up as processoxygen in the MBR tank 40 and partly released to atmosphere via evolvedbubbles. The zone 40 may be at about 10 ppm. Optional pre-formed seedbubbles and optional blower are indicated. In this system, as explainedabove, the supersaturation with oxygen occurs at the initial influent tothe system (as in FIGS. 9-11) rather than in an internal recycle in theMBR zone from 40 as in FIG. 3.

As an alternative, the influent can be fed directly to an MBR zone 40,with the influent treated with oxygen supersaturation and preferablyintroduction of seed bubbles, which would be at position 98 shown inFIG. 12. The MBR zone 40 can be a simultaneousnitrification/denitrification zone as in FIG. 3. This can have the sameadvantages as oxygenation of the recycle as in FIG. 3, and canpotentially have further advantages as described above. Such a systemwould be similar to FIG. 9, 10 or 11 without the recycle stream.

Supersaturating mixed liquor with oxygen drives up the DO concentrationin the liquid phase to 300 ppm or 30 times typical saturation levels andcan be roughly 80% more efficient than diffused aeration. The typicalair scour range 0.009-0.018 SCFM. This invention reduces air scourdemand to between 0.005-0.009 SCFM. The supersaturated dissolved oxygentends to remain in solution in the liquid, not significantly evolving asbubbles until something triggers their release, such as the seed bubblesdescribed herein.

The above described preferred embodiments are intended to illustrate theprinciples of the invention, but not to limit its scope. Otherembodiments and variations to these preferred embodiments will beapparent to those skilled in the art and may be made without departingfrom the spirit and scope of the invention as defined in the followingclaims.

1. In a sewage treatment system and process, a method for increasingefficiency in an MBR zone with membrane separators in which water isremoved from mixed liquor, wherein superoxygenation and air scour areemployed, comprising: supersaturating influent wastewater or mixedliquor with oxygen prior to biological treatment, so that the wastewateror mixed liquor is oxygenated beyond normal saturation content of water,to an oxygen content of at least 50 parts per million, introducing theoxygen-supersaturated wastewater or mixed liquor into a biologicalprocess zone requiring oxygen, thereby causing part of the oxygen in themixed liquor to be taken up to support the biological process, anddirecting the mixed liquor to membrane separators to remove water, andcausing essentially a remaining portion of supersaturated oxygen in themixed liquor to evolve from solution as relatively large bubbles whichform and rise from beneath the membrane separators with sufficientvelocity to cause air scour of the membrane separators in the MBR zone,whereby membrane air scouring requirements are reduced by the evolvingoxygen bubbles, while the dissolved oxygen can meet most of processoxygen demand.
 2. The method of claim 1, wherein the MBR zone comprisessaid biological process zone requiring oxygen.
 3. The method of claim 1,wherein said biological process zone requiring oxygen is a separate zoneupstream from the MBR zone.
 4. The method of claim 1, wherein saidbiological process zone requiring oxygen is an aerobic zone separate andupstream from the MBR zone.
 5. The method of claim 1, wherein the stepof causing supersaturated oxygen in the mixed liquor to evolve fromsolution is accomplished with an air blower releasing air bubbles belowthe membrane separators, the air bubbles acting as seed bubbles to causethe oxygen bubbles to evolve.
 6. The method of claim 1, wherein the MBRzone comprises said biological process zone requiring oxygen, andwherein the zone is maintained at a minimum level of oxygen availablefor the biological process so as to promote simultaneous nitrificationand denitrification in the zone.
 7. The method of claim 1, wherein theoxygen content of the supersaturated wastewater or mixed liquor isbetween 250 parts per million and about 350 parts per million.
 8. Themethod of claim 1, wherein the step of causing supersaturated oxygen inthe mixed liquor to evolve from solution is accomplished by introducingpre-formed seed bubbles into the oxygen-supersaturated wastewater ormixed liquor.
 9. The method of claim 8, wherein the injection ofpre-formed seed air bubbles is accomplished using air bubbles from adiffuser.
 10. In a sewage treatment system and process, a method forincreasing efficiency in an MBR tank in which water is removed frommixed liquor, and superoxygenation and air scour are employed,comprising: in a tank having membrane filtration units, recycling mixedliquor through an internal recycle stream and in the recycle streamsuper-oxygenating the mixed liquor beyond normal saturation content ofwater prior to the return of the mixed liquor to the tank, introducingthe recycle stream back into the tank generally beneath the membranefiltration units, to release oxygen bubbles, and injecting seed airbubbles into the mixed liquor in the recycle stream, to seed the oxygento cause the oxygen to evolve from solution as relatively large bubbleswhich form and rise with sufficient velocity to cause air scour of themembrane filtration units.
 11. The method of claim 10, wherein theinternal recycle stream has a flow rate at least twice the rate of flowof influent to the tank.
 12. The method of claim 10, wherein theinternal recycle stream has a flow rate at least four times the rate offlow of influent to the tank.
 13. The method of claim 10, wherein theinternal recycle stream has a flow rate at least five times the rate offlow of influent to the tank.
 14. The method of claim 10, wherein themixed liquor in the internal recycle stream is supersaturated to between50 and about 300 parts per million oxygen.
 15. The method of claim 10,further including promoting simultaneous nitrification anddenitrification in the tank by reducing oxygen content in the mixedliquor, alternating with superoxygenating the mixed liquor.
 16. Themethod of claim 10, without any air scour of the membrane filtrationunits other than bubbling occurring due to the superoxygenation and theseed air bubbles.
 17. The method of claim 10, wherein superoxygenationof the mixed liquor is conducted in a pressurized chamber.
 18. Themethod of claim 10, wherein the seed air bubbles constitute no more thanabout 50% of the volume of oxygen bubbles emerging out of solution. 19.The method of claim 10, wherein the seed air bubbles are introduced in aconduit carrying the recycle stream.
 20. The method of claim 10, whereinthe seed air bubbles are introduced below the membrane filtration unitsnear a point where the recycle stream is introduced back into the tank.21. In a sewage treatment system and process, a method for increasingefficiency in an MBR zone with membrane separators in which water isremoved from mixed liquor, wherein superoxygenation and air scour areemployed, comprising: supersaturating influent wastewater or mixedliquor with oxygen prior to biological treatment, so that the wastewateror mixed liquor is oxygenated beyond normal saturation content of water,to an oxygen content of at least 50 parts per million, introducing theoxygen-supersaturated wastewater or mixed liquor into a biologicalprocess zone requiring oxygen, thereby causing part of the oxygen in themixed liquor to be taken up to support the biological process, anddirecting the mixed liquor to membrane separators to remove water,whereby the dissolved oxygen can meet most of process oxygen demand.